Abigail was obviously physically involved with Proctor because of his reaction to her in Act I. She is flirtatious, which was not common in Puritan women. She was "let go" by the Proctor family after he admitted his adultery to his wife. Abigail is not welcome in their house. That is pretty clear even at this early point because of her corruption up to that point in time.
The previous relationship between Abigail and the Proctors was cordial until John and Abigail committed adultery and Elizabeth found out. Abigail was hired as a servant in the Proctor home, but she was fired when Elizabeth discovered the illicit relationship between her husband and Abigail. Elizabeth also stopped going to church in order to avoid contact with Abigail. Rumors about the affair reached Parris, and he questioned Abigail about the scandal, but she denied. John Proctor apologized to his wife and ended the relationship with Abigail. However, Abigail refused to end the relationship and tried to bewitch Elizabeth during the dance in the forest. She also implicated Elizabeth in witchcraft in attempts to have her hanged. She wanted Mrs. Proctor out of the picture so she could claim her husband.
Abigail Williams previously occupied the position currently held by Mary Warren in the Proctors' household. She would have helped with the cooking and washing, and the children, especially in light of Mrs. Proctor's illness following the birth of her last child. However, it is clear that Abigail and John Proctor had more than just a physical relationship. In Act One, when she insists that she has "a sense for heat [...] and [his] has drawn [her] to [her] window, and [she has] seen [him] looking up, burning in [his] loneliness," he admits to her that he "may have looked up" at her window. He further admits that "[he] may think of [her] softly from time to time." And although Proctor promises that he will not reach for her again, he has clearly admitted enough to confirm Abigail's belief that he loved her when Mrs. Proctor fired her and "[he does] now." Thus, their "prior relationship" is still, in some way, present.